Implantable medical devices provide therapy to treat numerous health conditions as well as monitoring and diagnosis. Over the years, the development of these devices has seen remarkable progress due to tremendous advances in microelectronics, electrode technology, packaging and signal processing techniques. Many of today's implantable devices use wireless technology to perform the dual functions of supplying power and providing communication. There are many challenges when creating an implantable device. Issues such as reliable and fast bidirectional data communication, efficient power delivery to the implantable circuits, low noise and low power for the recording part of the system, and delivery of safe stimulation to avoid tissue and electrode damage are some of the challenges faced by the microelectronics circuit designer.
Neuroengineering, the application of engineering techniques to understand, repair, replace, enhance, or otherwise exploit the properties of neural systems, is a topic that has garnered considerable interest in the research community. The nervous system is a complex network of neurons and glial cells. It comprises the central nervous system (brain and spinal cord) and the peripheral nervous system. Injuries or diseases that affect the nervous system can result in some of the most devastating medical conditions. Conditions, such as stroke, epilepsy, spinal cord injury, and Parkinson's disease, to name but a few, as well as more general symptoms such as pain and depression, have been shown to benefit from implantable medical devices. These devices are used to bypass dysfunctional pathways in the nervous system by applying electronics to replace lost function.
The first implantable medical devices were introduced in the late 1950s with the advent of the heart pacemaker and subsequently the cochlear implant. Both have restored functionality for hundreds of thousands of patients. A pacemaker uses electronics and sensors to continuously monitor the heart's electrical activity and when arrhythmia is detected, electrical stimulus is applied to the heart (via electrodes) to regulate its speed. A cochlear implant uses electronics to detect and encode sound and then stimulate the auditory nerve to enable deaf individuals to hear.
Implanted medical devices (IMDs) for multi-channel bio-signal recording require a high data rate for the uplink while being powered wirelessly. High-density recording of neural spikes, local field potential (LFP) or electrocoticogram (ECoG), etc., with more than 100 channels is required for advanced brain-computer interface (BCI) applications and research. One example for BCI implants is the flexible active ECoG array described by Viventi, et al. in Nature Neuroscience, Col. 14, No. 12, pp. 1599-1605, 2011, which is designed for subdural placement. The key elements of such implanted BCI devices are electrode, sensing, signal processing (analog and digital), data communication, and power harvesting and management. A key challenge with respect to power and data telemetry for this device, and others, comes from where it needs to be placed—within a very limited space in the body. Other obstacles include that the rate uplink telemetry requires a high data for transmission of recorded data. For example, ECoG with 1024 channels (at 600 S/s×10 bits) requires 6.15 Mbps, and Neural Spikes & LFP with 128 channels (at 5 kS/s×10 bits) requires 6.4 Mbps. This over 6 Mbps data rate is challenging because of its very limited amount of power, which is transferred wirelessly.
Possible solutions for power and data (especially uplink data) telemetry include single link and multiple link. The most viable solution for power transfer uses a pair of inductors, where data are also transferred through the same link for power transfer. Load shift keying (LSK), a widely used modulation scheme for uplink data telemetry, trades off power transfer and data-rate based on the coil's quality factor Q. This modulation scheme is based on the reflection of the implant's load to the transmitter via the inductive link. The binary data stream shorts the implant coil and the change in impedance is reflected in the transmitter because the implant load is much larger than the on-resistance of the switch transistor. High power transfer efficiency requires high Q, normally restricting the data rate due to the long rectification-off time. Where only two coils are used (for both power and data), there is a risk of disruption in power delivery if the short is applied for too long to the implant coil. When communication is in idle mode, the link should be optimized for maximum power transfer. The bandwidth of LSK is limited by the coupling factor, the parameters of the coils, and the transient response of the inductive link. Data rates of 100-500 kbps with simultaneous power transfer have been achieved by LSK, and a few Mbps using multiple dedicated inductive links for data transfer and power transfer. Using transient response from phase shifts by shorting the secondary LC tank for a half cycle has reportedly achieved 0.858-Mbps data rate with power transfer over single inductive link. However, this scheme loses energy whenever shorting the LC tank because of the subsequent reversal of LC resonance and the recovery time after transmitting one bit limits the data rate.
Although major advances have been achieved in the field of wireless communications and wireless powering for implants, further improvements in terms of new techniques that allow better optimization of the entire system are needed. Transceivers based on conventional wideband wireless radio technology are emerging. Approaches using higher RF bands require additional complexity in circuits and antenna structures, including sophisticated power management circuitry. These are expected to continue to offer improved performance in terms of an increase in output data rate with lower power consumption requirements, as smaller geometry silicon processing technologies are used for the implementation of the implantable circuits.